To provide paper pulp with its desired properties, such as proper brightness, the paper pulp is treated with one or more different treating fluids, for instance chlorine gas or oxygen. The pulp is therefore continuously conducted into a treatment device, in which the treatment fluid or fluids are brought into contact with the pulp fibers. In order to achieve optimal treatment results it is important that the fibers contact the treatment fluid as uniformly as possible, and for that reason fibers should be spread out to the greatest possible extent, and the treatment fluid should be homogeneously distributed in the suspension.
In prior so called low intensity devices the concentration of the treatment fluid of the pulp has been medium low to low in order to achieve the desired even quality of the fully treated pulp. These low intensity treatment devices have, to an ever increasing extent, been replaced by high intensity treating devices with a high concentration of the treatment fluid in relation to those previous types. Current devices of this latter type, however, unfortunately suffer from the drawback that the treatment zone, where the working medium undergoes shearing in order to mix the fibers and is subjected to the treatment fluid, does not show a completely homogenous shearing field. As a consequence, either (1) a part of the suspension will be incompletely treated, or (2) a part of the suspension will be "overtreated."
The treatment according to (1) results in a homogeneously treated final product, whereas a treatment according to (2) means that too much treating fluid must be supplied to the suspension and/or that unnecessarily high energy needs to be supplied to the pulp suspension.
Both of these alternatives add to the costs.
A treatment device of this type for treating pulp suspensions is disclosed in Swedish Patent No. 8001970-6, corresponding to U.S. Pat. No. 4,416,548. This device comprises an axial inlet for the pulp suspension, which opens into a mixing chamber, in which a rotor is arranged. The pulp is fed through and is sheared in an annular slot, which is delimited by a stator ring and the rotor. The supply fluid is conducted through an axial inlet to the plane end surface of the rotor and thereafter flows radially along this surface up to and into the shearing slot where it is mixed with the suspension. Thereafter, the treated pulp flows out through a tangential outlet arranged after the slot. As shown in FIGS. 7 and 8 of this Swedish publication, the shearing area may be divided into several annular shearing slots having the same radial extension formed by several rotor connected rotor rings, which are inserted between interconnected stator rings. Since the present invention to a substantial extent is based on this known technique, reference is made to this publication for a more detailed description of the principal construction. However, it should be noted that this known solution suffers from the above stated shortcomings with respect to inhomogeneous shearing fields.
The object of the present invention is to provide a method and apparatus of the above kind to treat a working medium by a treatment fluid, which provide a treatment zone with a shearing field which allows a substantially homogenous treatment of the working medium at a lower price than has heretofore been possible.